אָר֤וּר אַפָּם֙ כִּ֣י עָ֔ז וְעֶבְרָתָ֖ם כִּ֣י קָשָׁ֑תָה אֲחַלְּקֵ֣ם בְּיַעֲקֹ֔ב וַאֲפִיצֵ֖ם בְּיִשְׂרָאֵֽל׃ (ס)
פֿאַרשאָלטן זײער כּעס, װײַל ער איז שטאַרק, און זײער צאָרן, װײַל ער איז האַרט! איך װעל זײ צעטײלן אין יעקב, און זײ צעשפּרײטן אין ישׂראל.
Rashi
ארור אפם כי עז CURSED BE TIIEIR WRATH, FOR IT IS POWERFUL — Even when he was reproving them he did not curse them but their anger. That is what Balaam said: (Numbers 23:8) “How can I curse them since אל hath not cursed them?” (Genesis Rabbah 99:6)
Chizkuni
ארור אפם, “cursed be their anger;” this is not a curse, but a blessing. How so? Yaakov wishes them that whatever they will plan in anger should not succeed, so that they will learn the lesson not to act out of anger. An alternate exegesis of these words: Yaakov exclaims: “how disgraceful and despicable is their anger!”
Ibn Ezra
Cursed be their anger. He prophesied — or prayed — that their fiery tempers would cool down.